Printers, copiers and other types of image forming devices have become necessary productivity tools for producing and/or reproducing documents. Such image forming devices include, but are not limited to: desktop copiers, stand-alone copiers, scanners, facsimile machines, photographic copiers and developers, multi-function devices and other like systems capable of producing and/or reproducing image data from an original document, data file or the like.
In conventional xerography, electrostatic latent images are formed on a xerographic surface by uniformly charging a charge retentive surface, such as a photoreceptor. The charged area is then selectively dissipated in a pattern of activating radiation corresponding to the original image. The latent charge pattern remaining on the surface corresponds to the area not exposed by radiation. Next, the latent charge pattern is visualized by passing the photoreceptor past one or more developer housings comprising toner, which adheres to the charge pattern by electrostatic attraction. The developed image is then transferred to a receiving substrate, such as paper, to which it is fixed by a suitable fusing technique, resulting in a xerographic print or toner-based print.
It is known and customary to apply fuser oil to the fuser roll to provide the necessary release of a substrate from the fuser roll after the conventional toner image has been formed on the substrate. Fuser oils are known to one of ordinary skill in the art and include those disclosed in U.S. Pat. Nos. 7,198,875; 6,808,815; and 6,733,878, each of which is incorporated herein by reference in its entirety. As used herein, “substrate” refers to any output image receiving media that may be printed on, such as paper, pre-printed forms, transparency, cardboard, etc.
Fuser oils, such as non-functionalized or functionalized silicone oils, are useful for providing release of a substrate from a fuser roll found in an imaging device, such as in an electrophotographic device or an electrostatographic device. In such devices, some fuser oil may remain on the toner image, which may cover any portion of the substrate, and on the substrate itself. In other words, the fuser oil may at least partially cover a substrate having no toner image or a substrate having a toner image thereon. As used herein, “partially” refers to the release agent covering from about 1 percent to about 100 percent of the substrate, such as from about 10 percent to about 100 percent or from about 10 percent to about 90 percent of the substrate.
Thus, xerographic prints may include thereon a silicone fuser oil due to the printing process. In the case of amino functionalized fuser oil, the oil may chemically bond to the surface of the print because of hydrogen bonding between the amino component of the oil and the hydroxyl components in the substrate. The surface free energy (SFE) of xerographic prints containing amino functionalized silicone oil may dramatically drop from a range of higher than 30 mN/m2 to a range of from about 8 mN/m2 to about 30 mN/m2.
The presence of a fuser oil on the substrate, with or without a toner image thereon, can thus be detrimental to the ability of an adhesive to adhere to the substrate. Thus, applications such as print-on-demand book making are difficult because residual amino fuser oil resides on the print surface after fusing and interferes with glue and adhesive performance.
Fuser oils are commonly used in connection with various conventional toners, which have limits on acceptable exposure to elevated temperatures and pressure due to the Tg's (glass transition temperatures) of the resins comprising the toner. Unfortunately, this discourages using prints based on conventional, ultra low melt toners for applications such as print-on-demand car manuals, a market share for high-end car manufacturers.
The use of low Tg materials in some recent printing systems helps lower the energy necessary to produce a print, given that the energy consumption of normal xerographic equipment is quite high. Thus, xerographic equipment with lower power consumption has been designed. Toners which are designed to function in the lower power consumption equipment, known as “low-melt toners” , are made to have softening points of about 45° C. to about 65° C. However, an image defect known as document offset (or “blocking”) can occur at temperatures as low as about 45° C. to as high as about 70° C. or more when the toner begins to flow. Thus, low-melt toners often have a significant document offset problem. Document offset properties of various toners are set forth in Table 1.
TABLE 1Comparison of Document Offset Properties of VariousLow-Melt TonersMachineTemperature*DC2060 & DC12  62° C. (144° F.)DC40 & Majestik ® (Xerox Corp.)  61° C. (142° F.)DT18055.5° C. (132° F.)iGen3 ® (Xerox Corp.)55.5° C. (132° F.)*where Document Offset (DO) = 4.0 @ 10 g/cm2
As illustrated by Table 1, fused prints from these machines are limited to jobs that do not require the final product to be subjected to combinations of elevated temperature and pressure. This restriction is based upon the fact that the toner contains resins with characteristically low thermal glass transition temperatures, which when exceeded allow the resin to become amorphous and sticky. The stickiness of the toner results in prints that adhere to one another, either in an output tray or in the final product, and thus the prints become unusable. Unfortunately, the Tg's of these resins tend to be at or below temperatures that are easily achieved with day-to-day activities, such as car manuals in glove boxes.
In view of the energy consumption concerns mentioned above, there is a drive for toner to become ultra-low melt. Thus, the Tg's are anticipated to be lowered even more, which will result in even less robustness and image permanence under the above-mentioned environmental conditions.
Known methods of reducing document offset include adding wax to the toner itself (as in Emulsion Aggregation toner) and applying an overprint coating to the substrate. The overprint coating or varnish, either aqueous based or UV curable, is typically a liquid film coating that may be dried and/or cured. Drying may be accomplished through application of heat while curing may be accomplished by applying ultraviolet light or low voltage electron beams to polymerize (crosslink) the components of the overcoat. However, known overprint coatings, such as those described in U.S. Pat. Nos. 4,070,262; 4,071,425; 4,072,592; 4,072,770; 4,133,909; 5,162,389; 5,800,884; 4,265,976; and 5,219,641, for example, fail to adequately protect xerographic prints and fail to reduce document offset.
Additionally, the above described methods indicate that a coating may be applied to the surface of the substrate, with an image thereon, to cover the surface during a finishing step. The coating covers the entire surface of the substrate to protect the toner image from being rubbed from or scratched from the surface of the substrate. The coating may be a continuous dry film that is formed over the image and substrate. Digital Application of spot coating the image only component of the substrate is not possible due to the high viscosities of the coatings.
A problem observed with unfused toner images is that the output image receiving media exiting the marking module, where electrostatically charged toner particles are deposited on the substrate, must be very carefully handled because unfused toner is susceptible to distortion if subjected to any physical disturbance.
As used herein the term “unfused” is used to describe the condition of an output image receiving media or substrate to which an image forming substance, such as toner, has been applied in the formation of a copy of an original image. The unfused image may include text and/or graphics and the toner has not yet been fixed, generally by some form of heat and/or pressure fusing. The term “partial fusing” refers to a process of heating the toner to a temperature just below the melting point of the toner such that the toner becomes sticky and adheres to the substrate (no pressure is applied to congeal the toner particles together). A substrate with an “unfused” toner image is particularly susceptible to image degradation based on rubbing or smearing.
Because coatings of previous methods typically cover the entire surface of the substrate, the coating may often enhance the gloss of the surface, which may increase the visual appeal of the print or image, depending on the customers needs. If the coating is removed from the surface of the substrate, the continuous film formed by the coating may become non-uniform or non-continuous across the surface of the substrate. As a result, the coating removed from the surface may form one or more visual defects to the gloss or to the continuous film.
In addition, known coating formulations fail to prevent the formation of creasing or hairline cracks on the print surface in response to thermal expansion of the toner, which creates an undesirable appearance. This is a particularly important issue for automobile manuals, book covers, etc., which require the prints therein to survive high temperatures for hours at a time, yet retain a uniform appearance.
Therefore, a need exists for a system and a method for selectively protecting toner images on the surface of a substrate. Additionally, a need exists for a system and a method for protecting toner images with a coating which may increase the ability of the print to resist blocking, thereby improving the robustness of the print. Further, a need exists for a system and a method that applies heat and/or pressure to a partially fused image and coating for maintaining image integrity. Moreover, a need exists for a system and a method that provides a coating to minimize damaging effects to the final image caused by document offset or blocking.
Furthermore, a need exists for a protective coating composition that provides coating properties including, but not limited to: reduction or prevention of document offset, as well as protection of an image from sun, heat and smearing, particularly in commercial print applications.